Central obesity/insulin resistance (OB/IR), is associated with a clustering of cardiovascular risk factors. OB/IR is also associated with impaired endothelial dependent vasodilation (EDV) characterized by reduced nitric oxide (NO) release. Endothelial dysfunction and NO deficiency are potent mechanisms for macrovascular disease. While insulin enhances EDV in insulin sensitive subjects, insulin paradoxically reduces EDV in OB/IR subjects, suggesting an effect of IR to alter the balance between vasoconstrictor and vasodilator forces. The overarching hypothesis of this proposal is that OB/IR is associated with an imbalance between the endothelial production of the potent vasodilator NO and vasoconstrictor endothelin 1 (ET1), which contributes to abnormal vascular wall biology, vascular tone dysregulation and blood pressure elevation. The primary goal is to determine the role of ET1 in the endothelial dysfunction observed in the human IR. To this end, we will address the following questions: 1-Does excessive ET1 action contribute to impaired EDV in OB/IR subjects? Lean control, OB non-diabetic, and type 2 diabetic subjects will be studied. Basal leg vascular tone and endothelium dependent and independent vasodilation will be assessed with intra-arterial infusion of methacholine chloride and sodium nitroprusside, respectively, during infusion of saline or the endothelin A (ET A) receptor antagonist BQ-123. It is expected that ET A antagonism will at least partially restore endothelial function in insulin resistant subjects; 2-Does ET1 action contribute to impaired insulin mediated vasodilation and glucose uptake in OB/IR? In similar subject groups as in (1), leg glucose uptake, insulin mediated vasodilation, circulating ET1 levels and EDV will be assessed during euglycemic hyperinsulinemic clamp studies, with and without intra-arterial co-infusions of the ET A receptor antagonist BQ-123. We expect that ET A antagonism will ameliorate insulin action on both the vasculature and on glucose metabolism; 3-Does ET1 play a role in free fatty acid (FFA) induced endothelial dysfunction? Elevation of FFA causes IR, endothelial dysfunction, and impaired insulin mediated vasodilation. We hypothesize that FFA's effect to alter EDV is mediated by an increase ET1 release. In lean insulin sensitive subjects, we will assess endothelial function during elevations of circulating FFA achieved via the infusion of intralipid and heparin, with and without intra-arterial infusions of the ET A receptor antagonist BQ-123; 4-Is coronary flow reserve impaired in insulin resistant subjects? 13N ammonia positron emission tomography will be used to compare coronary flow reserve (CFR) in young healthy insulin sensitive and OB insulin resistant subjects. To assess the effects of IR and ET1, independent of coronary risk factors associated with OB/IR, we will also assess CFR in subjects rendered insulin resistant via a prolonged FFA elevation +/- ET A antagonism. This proposal will elucidate the role of ET1 in the endothelial dysfunction observed in OB/IR. If the data confirm our hypothesis, treatment of IR patients with ET1 antagonists may be a rational and beneficial approach to reducing the dreaded cardiovascular outcomes associated with IR and type 2 diabetes.